1. Field of the Invention
This invention relates to curable epoxy powder compositions, catalysts for curing powder coating epoxy compositions and to a process for curing epoxy powder coatings.
2. Description of the Prior Art
In recent years, there has been a substantial amount of work performed in the development of powder coating compositions for application to a variety of substrates. The use of powder coatings overcomes many of the environmental problems liquid, resinous coating compositions have presented in manufacturing, particularly where a vinyl organic solvent is used as the carrier.
Essentially, two methods of formulating epoxy powder coating systems have been used. One method is to grind the epoxy powder coating and combine it with various additives, pigments and catalysts in a blender and then apply this physical blend to a substrate and cure at elevated temperature, e.g. 270.degree. F.-400.degree. F. The second technique has been to melt mix the epoxy resin with various additives, pigments and catalysts and extrude this melt mix into a thin ribbon and cooling to form a solid. This solid is then ground into a finely divided form, e.g. 250 microns or less, depending upon whether the coating is to be used for decorative or functional purposes. The latter method of formulating epoxy powder coatings has significant advantages in that it permits greater uniformity of dispersion of additives, pigments and catalysts within the resin itself. It also presents the most difficulty in formulation due to the fact that many catalysts are too reactive at the melt mix temperature or else have a high activation temperature thus requiring a high curing temperature.
There are number of patents which disclose the use of various catalyst systems for epoxy powder coatings. Representative of these patents are:
Johnson, U.S. Pat. Nos. 3,519,576 and 3,520,905, which disclose the use of a heat activatable curing agent consisting of a crystalline polyphenate salt of a polyamine and a polyhydric alcohol, e.g. the salt of bis-phenol A and triethylenetetramine and the N,N-dimethyl-1,3-propanediamine salt of trimethyltrihydroxyflavan as a latent catalyst for epoxy powder coatings.
Aelony, U.S. Pat. No. 3,352,775 discloses the use of an amine salt of an epoxy resin as a catalyst for curing epoxy resins.
Heer et al U.S. Pat. No. 3,629,181 discloses curing agents of primary diamines, e.g. isophorone diamine with an excess of polyglycidyl ether as a curing agent for epoxy powder coatings.
Winfield et al U.S. Pat. No. 3,284,407 discloses the use of particular accelerators for epoxy resin compositions, the accelerator consisting of a substituted ammonium phenate prepared by heating a phenol with a tertiary amine or quaternary ammonium hydroxide. Suggested accelerator possibilities include the reaction product of bis-phenol A and various tertiary amines, e.g. triethyl amine and triethanol amine.